Such a type of press-punch retainer device is the device intended for selecting, from among a plurality of punch-die sets mounted on a single press machine for sheet metal processing, a particular set of punch-dies to be used according to the type of workpiece. In other words, the amount of projection of a punch, selected for processing, from the retainer device is increased, whereby the selected punch is brought into abutment against a workpiece. On the other hand, the amount of projection of the remaining non-selected punches is reduced, whereby none of them are brought into abutment against the workpiece.
One such retainer device is disclosed in Japanese Patent Kokai Publication No. (2001)18017. This retainer device comprises: a punch holding body for holding a punch; a retainer block to which the punch holding body is interfitted movably in a vertical direction; a spring for biasing the punch holding body into a non-use position of decreasing the amount of projection of the punch; a cam plate having at its forward end a cam part which is brought into abutment against the punch holding body; and a cylinder device by which the cam plate is drawn backward and forward in a horizontal direction. The cam plate is connected, at its base end, to the forward end of a piston rod of the cylinder device via a support.
When the cam plate is drawn forward, a cam part at the forward end of the cam plate comes into abutment against an inclined surface of the punch holding body and slides on the inclined surface, whereby the punch holding body is depressed downward in opposition to the biasing force of the spring. As the result of this, the amount of projection of the punch from the retainer block is increased. On the other hand, when the cam plate is brought backward, the punch holding body is brought into its original position by the biasing force of the spring. As the result of this, the amount of projection of the punch is decreased.
In the above-described retainer device, the punch holding body is so biased by the spring as to be brought back to its non-use position. However, the spring may get twisted by deterioration or due to frictional force developed by friction between the punch holding body and the retainer block. In such a case, there is the possibility that the punch holding body remains descended even when the cam plate is brought backward and, as a result, the punch fails to return to the non-use position. Accordingly, if pressworking is carried out when a punch unnecessary for processing is not yet brought back to the non-use position, this gives rises to the problem that a workpiece is damaged therefore resulting in the production of a defective product.